System and method for measuring power of optical signals carried over a fiber optic link

ABSTRACT

A pilot tone generator receives optical energy from an optical communication medium carrying a plurality of optical signals. Each optical signal carries data modulated at a unique wavelength and further modulated with a unique identification signal. The identification signal has an amplitude corresponding to an optical power of the associated optical signal. The pilot tone receiver detects each identification signal from the optical energy received and determines its corresponding amplitude. The pilot tone receiver calculates the optical power of each optical signal in the optical energy in response to the amplitude of the associated identification signal.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/300,310 filed Jun. 22, 2001.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in genaral to optical networks and moreparticularly to a system and method for measuring power of opticalsignals carried over a fiber optic link.

BACKGROUND OF THE INVENTION

In a wavelength division multiplexing (WDM) optical system, it isdesirable to measure optical powers of individual optical signalstransported along a fiber optic link. Conventional methods of performingsuch power measurements require expensive components to separate theoptical signals transported in the fiber prior to power measurement sothat each signal may be measured individually. Not only are theyexpensive, these optical components tend to be physically bulky and addto the considerations during management of the fiber optic link.

SUMMARY OF THE INVENTION

From the foregoing, it may be appreciated by those skilled in the artthat a need has arisen for a technique to measure power of opticalsignals transferred over a fiber optic link. In accordance with thepresent invention, a system and method for measuring power of opticalsignals carried over a fiber optic link are provided that substantiallyeliminate or greatly reduce disadvantages and problems associated withconventional optical power measurement techniques.

According to an embodiment of the present invention, there is provided amethod for measuring power of optical signals carried over a fiber opticlink that includes receiving a plurality of optical signals from thefiber optic link with each optical signal including an identificationsignal modulated therewith. An amplitude of each identification signalreceived is determined and an optical power of each optical signal isdetermined in response to the amplitude of each identification signalreceived.

The present invention provides various technical advantages overconventional optical power measurement techniques. For example, onetechnical advantage is in the simultaneous measurement of optical powerof a plurality of optical signals without separating the optical signalsfor individual measurement. Another technical advantage is in the use ofless costly and reduced number of optical components since only themodulations of many optical signals are detected and analyzed. Yetanother technical advantage is to adjust a detection bandwidth accordingto receiver position in the network, signal to noise ratio of receivedoptical signals, and/or desired accuracy. Other technical advantages maybe readily ascertainable by those skilled in the art from the followingfigures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals represent like parts, in which:

FIG. 1 illustrates a simplified block diagram of an optical network;

FIG. 2 illustrates a simplified block diagram of a pilot tone receiverin the optical network;

FIG. 3 illustrates a process flow diagram for measuring optical power ofoptical signals received at the pilot tone receiver and transported inthe optical network.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified block diagram of an optical network 10. Opticalnetwork 10 includes a transmitter 12 that receives data inputs I₁–I_(n).Transmitter 12 includes a plurality of laser and pilot tone modulatorunits 12 a–12 n. Laser and pilot tone modulator unit 12 a generates anoptical signal 14 a at a unique predetermined wavelength λ₁ to transportinformation received at data input I₁. Laser and pilot tone modulatorunit 12 a also modulates a unique identification signal, or pilot tone,ID₁ onto its generated optical signal 14 a. Similarly, other laser andpilot tone modulator units, such as 12 n, generate optical signals 14 nwhere the associated data input I_(n) is modulated at a uniquepredetermined wavelength λ_(n) and is modulated with a uniqueidentification signal ID_(n).

Optical network 10 includes a combiner 15 operable to receive aplurality of optical signals 14 a–14 n and to combine those signals intoa multiple wavelength signal 16. As one particular example, combiner 15may be a wavelength division multiplexer (WDM). Optical network 10communicates multiple wavelength signal 16 over an optical communicationmedium 20. Optical communication medium 20 may have a plurality of spans20 a–20 n of fiber, each with an optical amplifier 22 or other types ofoptical elements including an optical add/drop multiplexer, an opticalcross connect unit, signal conditioning devices, and/or lossy elements.One type of optical element used in optical network 10 may be an opticaltap 24. Optical tap 24 allows for a distant location to capture aportion of the transmission carried by optical communication medium 20.

Optical network 10 also includes a separator 26 operable to separateindividual optical signal 14 a–14 n from multiple wavelength signal 16.Separator 26 can communicate individual signal wavelengths or ranges ofwavelengths to a bank of receivers 28 a–28 n and/or other opticalcommunication paths. Separator 26 may be, for example, a wavelengthdivision demultiplexer (WDM). Receivers 28 a–28 n receive respectiveoptical signals 14 a–14 n for decoding in order to recover the originalsignal as a respective data output O₁–O_(n).

In order to manage optical network 10, it is desirable to measure theoptical power of each optical signal 14 a–14 n carried over opticalcommunication medium 20. The present invention contemplates the use ofidentification signals ID₁–ID_(n) to measure the optical power of theircorresponding optical signal 14 a–14 n.

FIG. 2 shows a simplified block diagram of a pilot tone receiver 30 usedto measure optical power of optical signals 14 a–14 n. The functionsperformed by pilot tone receiver 30 may be performed in hardware,software, or a combination of both. Pilot tone receiver 30 receives aportion of the optical transmission from optical communication medium 20through optical tap 24. Optical tap diverts a portion of the opticalenergy from optical communication medium 20 to pilot tone receiver 30.In the example shown, optical tap 24 extracts 5% of the optical energyfrom optical communication medium 20 though other percentages ofextraction may be incorporated as desired.

Pilot tone receiver 30 receives the extracted optical energy of opticalsignals 14 a–14 n from optical tap 24 at an optical/electrical converter32. Optical/electrical converter 22 converts the optical energy intoelectrical signals. The electrical signals are fed to an anti-aliasfilter 34 for removal of the high frequency component of the electricalsignals. The anti-aliased electrical signals are then combined at acombiner 36 with random noise from a dither source 38 to improve thesignal to noise quality of the original signals. The improved electricalsignals are then converted into digital form by an analog to digitalconverter 40.

A filtering and down sampling unit 42 performs several functions on thedigital signals received from analog to digital converter 40. Band passfilters may be used in order to isolate the frequencies of interest inthe digital signals. The data rate of the digital signals may also bedown sampled to minimize processing, allow long time storage, and allownarrow detection bandwidths. Filtering and down sampling unit 42 thendetects for each identification signal ID₁–ID_(n), either sequentiallyor in any desired order through changing of identification signaldetection coefficients, and measures its amplitude for processing by anoptical power processor 44. Optical power processor 44 storesinformation from the detected identification signal in a working storage46 to perform the appropriate processing and coordinates withinformation about pilot tone receiver 30 determined at manufacture andstored in a calibration storage 48. Optical power processor 42determines an optical power of an associated optical signal from theamplitude of its identification signal. The measured optical power maythen be used to adjust any amplifier gains within optical network 20 asdesired.

Identification signals ID₁–ID_(n) may be of a variety of types providedthat, when detected, their amplitude is proportional to the opticalpower of the associated optical signals 14 a–14 n. Amplitude modulationis one technique for providing the appropriate proportionality. It isalso preferable for identification signals ID₁–ID_(n) to not interferewith one another during transport and detection. With no interference,the optical power of many optical signals may be measured simultaneouslywithout individually separating out the optical signals. Thisrequirement can be accomplished through sine wave amplitude modulationwith different frequencies for each of identification signalsID₁–ID_(n). Since only a small amount of the optical energy is extractedby optical tap 24 from optical communication medium 20, only smallamplitudes of modulation are used for identification signals ID₁–ID_(n).As an example, a 4% amplitude modulation may be performed foridentification signals ID₁–ID_(n). By using small amplitudes ofmodulation, identification signals ID₁–ID_(n) do not interfere with thedata traffic carried by optical signals 14 a–14 n. Through sine waveamplitude modulation detection, the optical power for a given opticalsignal 14 is determined as follows:P=R/(M*G),

-   -   where P is the optical power to be measured,        -   R is the amplitude of the received identification signal,        -   M is the index of modulation used to modulate the optical            source with the identification signal, and        -   G is the gain of the modulation receiver.

The accuracy of the measurement of P depends on the accuracy of each ofR, M, and G. The accuracy of R, the amplitude of the identificationsignal, relates to the signal to noise environment at the detectionpoint. At a location with many optical signals 14 a–14 n present, anoise density level N per Hertz is controlled by the aggregate of theseoptical signals 14 a–14 n. The noise in the detection bandwidth B thenbecomes N*B. The ratio of signal S to noise in the detection bandwidthis thus S/(N*B). If an arbitrarily small optical signal 14 is present atthe location, it may have an arbitrarily low signal to noise densityratio S/N. To achieve a specified accuracy for R, it will be required toachieve a minimum signal to noise ratio in the detection bandwidth.Thus, for small signals S, bandwidth B will be minimized to achieve aspecified accuracy for R.

The accuracy of M, the modulation index, is dependent upon how preciselythe source modulation is known and how the modulation index changesduring optical signal 14 propagation. A technique for preciselycontrolling M at the optical source can be found in copending U.S.patent application Ser. No. 09/567,576 filed May 10, 2000 and entitled“Method and Apparatus for Maintaining a Pre-determined Ratio of a PilotTone Power and a Mean Optical Output Power of an Optical Signal” whichis hereby incorporated herein by reference. The variation of themodulation index during propagation is mostly dependent on an amount ofamplified spontaneous emission noise included in the optical channelmeasurement. The amount of amplified spontaneous emission noise isrelated to optical bandwidth. However, with good optical carrier tonoise ratios where bit error rates are less than 10⁻¹², the variation ofthe modulation index with propagation is generally negligible.

The accuracy of G, the modulation receiver gain, depends upon how wellthis parameter is known. Pilot tone receiver 30 provides a quantifiableoutput R sensitive to a particular identification signal using variousoptical and electronic components. The major uncertainty of G is in thevariability of the operating characteristics of each optical andelectrical component from one unit to the next, especially the unit tounit variability of optical taps 24. Being unit to unit related, thisvariability can be measured in conjunction with all components of pilottone receiver 30 at time of manufacture and included as calibration datastored in calibration storage 48. The accuracy of G then becomesdependent upon how well it is measured at the time of manufacture and ifit drifts with time and environment during use.

As seen in pilot tone receiver 30, detection of R occurs in the digitaldomain and in the program domain. This allows for an ability to vary thedetection bandwidth. By being able to vary the detection bandwidth,pilot tone receiver 30 may be optimized for the signal to noise ratiopresent at a particular detection point within optical network 10. Asoptical signals propagate through optical network 10, the signal tonoise ratio seen by pilot tone receiver 30 changes as optical channelsare added or dropped from any given optical span 20 a–20 n. For a givenaccuracy of R at different detection points within optical network 10,different detection bandwidths may be implemented. Also, if at a givendetection point a different accuracy of R is desired, the detectionbandwidth may be varied to accommodate the new accuracy requirement.Variation of the detection bandwidth may be performed on an opticalsignal by optical signal basis.

With sine wave amplitude modulation used for identification signalsID₁–ID_(n), an example limit on the narrowness of the detectionbandwidth may be the sum of the phase noise of the modulationtransmitter 12 and the phase noise at the frequency reference of pilottone receiver 30. The phase noise of the modulation transmitter 12controls how wide the frequency is for the identification signalID₁–ID_(n) modulation. The phase noise at the frequency reference ofpilot tone receiver 30 controls a minimum detection bandwidth. Anadditional limitation on the narrowness of the detection bandwidth isthe amount of space allocated in working storage 46 to perform filteringat the detection bandwidth. Signal to noise ratio improves as moreinformation is accumulated and stored for processing. As bandwidthbecomes small, the sample time desired increases. Storage requirementsfor longer sample times is larger than for shorter sample times. Highsample rates may require a relatively large amount of storage space.Down sampling performed by filtering and down sampling unit 42 slows therate that information leaves analog to digital converter 40 so that lessstorage space is needed for processing. This limitation may beinsignificant if sufficient storage space can be provided in pilot tonereceiver 30.

One of the requirements for components within optical network 10 relatesto reliability. Certain components that carry large numbers ofwavelengths and thus large amounts of data traffic should be designedwith high reliability characteristics. Reliability in electronic systemscan be maximized in several ways. One way is to minimize hardware andthe other way is to minimize software. Lots of hardware or lots ofsoftware are well known to lead to reliability problems. By selectingthe component interface for traffic critical components at the junctionbetween filtering and down sampling unit 42 and optical power processor44, reliability is maximized without any compromise to accuracy. Anycomponent interface selected prior to analog to digital converter 40 canlead to degraded accuracy due to the added complexity of conveying ananalog value across the boundary. A component interface between analogto digital converter 40 and filtering and down sampling unit 42 leads toless reliability due to the relatively high data rate for data acrossthis boundary. Selecting the component interface after optical powerprocessor 44 has less reliability due to the inclusion of software andthe electronics associated with the processing of the power measurement.

FIG. 3 shows an example process flow diagram for measuring power of anoptical signal. The process begins at block 50 where each optical signal14 a–14 n is modulated with a unique identification signal. Eachidentification signal ID₁–ID_(n) is modulated with an amplitudeproportional to an optical power of its associated optical signal 14a–14 n. The optical signals 14 a–14 n are multiplexed for transmissionacross optical communication medium 20 at block 52. A portion of theoptical energy transmitted across optical communication medium 20 isextracted at block 54 by optical tap 24. At block 56, the optical energyis converted to electrical signals. At block 58, the high frequencycomponents within the electrical signals are removed. At block 60, thesignal to noise ratio of the electrical signals is improved. A digitalrepresentation of the electrical signals is generated at block 62. Atblock 64 the detection bandwidth is determined. At block 66, filteringis performed according to the detection bandwidth. The data rate of thedigital representation is down sampled to a lower rate at block 68. Anidentification signal is detected at block 70 and its amplitude isdetermined at block 72. An optical power of the associated opticalsignal is calculated at block 74 in response to the amplitude of theidentification signal. The process is repeated for each optical signaland identification signal pair carried by optical communication medium20. In this manner, optical power of an optical signal is determinedwithout having to process any of the data carried by the optical signal.

Thus, it is apparent that there has been provided, in accordance withthe present invention, a system and method for measuring power ofoptical signals carried over a fiber optic link that satisfies theadvantages set forth above. Although the present invention has beendescribed in detail, it should be understood that various changes,substitutions, and alterations may be made herein. For example, pilottone receiver 30 may include other or fewer functions than those shownand described and still measure the optical power of an optical signalusing a detected amplitude of its identification signal. Other examplesmay be readily ascertainable by those skilled in the art and made hereinwithout departing from the spirit and scope of the present invention asdefined by the following claims. Moreover, the present invention is notintended to be limited in any way by any statements or any example madeabove that is not otherwise reflected in the appended claims.

1. A method for measuring power of optical signals carried over a fiberoptic link, comprising: receiving a plurality of optical signals fromthe fiber optic link, each optical signal including an identificationsignal modulated therewith and different from any other optical signal;determining an amplitude of each identification signal received, whereinthe amplitude of each identification signal received is determinedwithout processing any information carried by the optical signal;determining an optical power of each optical signal in response to theamplitude of each identification signal received.
 2. The method of claim1, wherein the identification signals do not interfere with data carriedby the optical signals.
 3. The method of claim 1, wherein eachidentification signal does not interfere with any other identificationsignal.
 4. The method of claim 3, wherein each identification signal issine wave amplitude modulated to its respective optical signal.
 5. Themethod of claim 1, wherein only a portion of the total energy of theplurality of optical signals is received.
 6. The method of claim 1,wherein the optical power of each optical signal is proportional to theamplitude of its respective identification signal.
 7. The method ofclaim 1, further comprising: down sampling a data rate of the opticalsignals to provide narrow detection bandwidths.
 8. The method of claim1, further comprising: adjusting a detection bandwidth at a detectionpoint of the optical signals.
 9. A method f or measuring power ofoptical signals carried over a fiber optic link, comprising: receiving aplurality of optical signals from the fiber optic link, each opticalsignal including an identification signal modulated therewith;determining an amplitude of each identification signal received;determining an optical power of each optical signal in response to theamplitude of each identification signal received; adjusting a detectionbandwidth at a detection point of the optical signals, wherein thedetection bandwidth is determined in response to a phase noise of atransmitter used in modulating the identification signals onto theoptical signals and a phase noise of a frequency reference used inreceiving the optical signals.
 10. The method of claim 1, furthercomprising: determining an amount of accuracy required on the amplitudeof each modulation signal.
 11. A system f or measuring power of opticalsignals carried over a fiber optic link, comprising: a filtering anddown sampling unit operable to receive a digital representation ofoptical signals carried by an optical transmission medium, the filteringand down sampling unit operable to identify an identification signalassociated with a particular one of the optical signals, the filteringand down sampling unit operable to determine an amplitude of theidentification signal without processing any information carried by theoptical signal; an optical power processor operable to receive theamplitude of the identification signal from the filtering and downsampling unit, the optical power processor operable to determine anoptical power of the particular optical signal in response to theamplitude of the identification signal.
 12. A system for measuring powerof optical signals carried over a fiber optic link, comprising: afiltering and down sampling unit operable to receive a digitalrepresentation of optical signals carried by an optical transmissionmedium, the filtering and down sampling unit operable to identify anidentification signal associated with a particular one of the opticalsignals, the filtering and down sampling unit operable to determine anamplitude of the identification signal; an optical power processoroperable to receive the amplitude of the identification signal from thefiltering and down sampling unit, the optical power processor operableto determine an optical power of the particular optical signal inresponse to the amplitude of the identification signal; wherein thefiltering and down sampling unit is operable to adjust a detectionbandwidth used in identifying the identification signal.
 13. The systemof claim 12, wherein the filtering and down sampling unit is operable todecrease a data rate of the digital representation to narrow thedetection bandwidth.
 14. The system of claim 11, wherein theidentification signals for the optical signals do not interfere witheach other to allow the optical power of the optical signals to bemeasured simultaneously.
 15. A system for measuring power of opticalsignals carried over a fiber optic link, comprising: a filtering anddown sampling unit operable to receive a digital representation ofoptical signals carried by an optical transmission medium, the filteringand down sampling unit operable to identify an identification signalassociated with a particular one of the optical signals, the filteringand down sampling unit operable to determine an amplitude of theidentification signal; an optical power processor operable to receivethe amplitude of the identification signal from the filtering and downsampling unit, the optical power processor operable to determine anoptical power of the particular optical signal in response to theamplitude of the identification signal; a calibration storage havingcalibration data used by the optical power processor in determining theoptical power of the optical signals.
 16. A computer readable mediumhaving code for measuring power of optical signals carried over a fiberoptic link, the code operable to: receive a plurality of opticalsignals, each of the plurality of optical signals having a uniqueidentification signal modulated therewith, each unique identificationsignal having an amplitude corresponding to an optical power of itsassociated optical signal; adjust a detection bandwidth for each uniqueidentification signal according to a desired power measurement accuracy;identify each unique identification signal within its associateddetection bandwidth; determine the amplitude for each uniqueidentification signal; calculate an optical power of each optical signalfrom the determined amplitudes.
 17. The computer readable medium ofclaim 16, wherein the code is further operable to: down sample a datarate associated with the plurality of optical signals in order to narrowthe detection bandwidth.
 18. The computer readable medium of claim 16,wherein the code is further operable to: limit a width of the detectionbandwidth in response to a phase noise associated with transmission ofthe plurality of optical signals and a phase noise associated withreceipt of the plurality of optical signals.
 19. The computer readablemedium of claim 16, wherein the code is further operable to: vary thedetection bandwidth in response to a signal to noise ratio at a point ofreceipt of the plurality of optical signals.
 20. The computer readablemedium of claim 16, wherein the code is further operable to: calculatethe optical power of the optical signals in response to a desiredaccuracy of the identification signal amplitudes, a desired accuracy ofthe modulation index associated with the identification signals, and adesired accuracy of the gain at a detection point of the identificationsignals.